Photophysical divergence driven by π-spacer variations in the anthracene-cyanostilbene architecture

A series of four anthracene-incorporated cyanostilbene derivatives with donor-π-acceptor (D-π-A) architectures was synthesized to investigate the effect of a phenyl π-spacer. Compounds without the spacer (2a, 2b) showed low quantum yields (Φ_F = 0.05-0.25) in various solvents, negative solvatochromism, aggregation-induced enhanced emission (AIEE), and red-shifted mechanofluorochromism. In contrast, those with the spacer (3a, 3b) exhibited high quantum yields (Φ_F = 0.75-0.85), positive solvatochromism, aggregation-induced emission (AIE), and blue-shifted mechanofluorochromism, highlighting the key role of the spacer in modulating photophysical properties. Quantum chemical calculations reveal that the compounds incorporating a phenyl π-spacer exhibit conformational flexibility between syn- and anti-conformers in both the ground and excited states, due to an energetically more accessible transition state relative to analogues lacking the spacer. The findings in this study underscore the importance of strategic structural engineering in anthracene-based cyanostilbene systems, which profoundly influences their photophysical properties, paving the way for their potential use in future optoelectronic applications.